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Measurement of the Evolution of Reactor Antineutrino Flux and Spectrum at Daya Bay Phys. Rev. Lett. 118, 251801 David Martinez Caicedo Illinois Institute of Technology on behalf of Daya Bay Collaboration The 26th International Workshop on

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SLIDE 1

Measurement of the Evolution of Reactor Antineutrino Flux and Spectrum at Daya Bay

David Martinez Caicedo Illinois Institute of Technology

  • n behalf of Daya Bay Collaboration

The 26th International Workshop on Weak Interactions and Neutrinos

June 20th 2017

  • Phys. Rev. Lett. 118, 251801
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SLIDE 2

Reactors: Great Antineutrino Source

2

  • Nuclear reactors are a powerful νe source.
  • More than 99 % of νe are the fission products of 235U, 239Pu,

241Pu, 238U.

  • fission/second per (~6 νe per fission)
  • G. Zeller, J. Formaggio

2 × 1020

GWth

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SLIDE 3

David Martinez - IIT

3

Reactor antineutrino anomaly

  • Previous experiments could have been biased to report

flux measurements that agreed with existing predictions

  • f the time
  • Probably attributable to uncertainties in beta to νe

conversion

  • The deficit could result from short base line sterile

neutrino oscillations Big question: Do we have a reactor antineutrino anomaly?

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SLIDE 4

David Martinez - IIT

Reactor antineutrino anomaly

  • The previous experiments could have been biased to

report flux measurements that agreed with existing predictions of the time: NO

  • Daya Bay also see the reactor flux deficit: ~5.4%

deficit relative to 2011 Huber/Mueller flux prediction.

  • Blind analysis: no reactor power data available until

the analysis is totally fixed

4

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SLIDE 5

David Martinez - IIT

Reactor antineutrino anomaly

  • Probably attributable to uncertainties in beta to νe conversion: YES
  • Prompt energy spectrum disagree with predictions.
  • If measured spectrum does not match, why should measured flux?

5

RENO, Neutrino2016

Daya Bay, Chin. Phys. C 41(1) (2017)

Double Chooz

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SLIDE 6

David Martinez - IIT

  • The deficit could result from short base

line sterile neutrino oscillations: YES

  • Consistent with hints of 1 eV sterile

neutrinos (LSND, MiniBooNE, Gallex)

  • In order to interpret CP violation results

we need to know if sterile neutrinos exist.

  • !DUNE needs the anomaly explanation!

6

Reactor antineutrino anomaly

10.1007/JHEP11(2015)039

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SLIDE 7

David Martinez - IIT

Reactor antineutrino anomaly: Recap

  • Reactor flux model predictions are not totally

correct

  • eV scale sterile neutrinos exist
  • Need more information to determine which of

these hypothesis (or both) are correct!

7

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SLIDE 8

David Martinez - IIT

Daya Bay Layout

8

  • Original concept with 


8 ‘identical’ detectors:

  • Near detectors 


constrain flux.

  • Far detectors see if


any neutrinos have
 disappeared.

  • Daya Bay has ideal


features for doing this!

! ! ! !Reactor![GWth] !Target![tons] ! !Depth![m.w.e]!

!

Double!Chooz! !!!8.6! ! ! !!!16!(2!×!8) ! !300,!120!(far,!near)! RENO ! ! !16.5! ! ! !!!32!(2!×!16) ! !450,!120! Daya!Bay! ! !17.4! ! ! !160!(8!×!20) ! !860,!250!! Large Signal! Low Background!

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SLIDE 9

David Martinez - IIT

The Daya Bay antineutrino detector

  • Detect inverse beta decay with liquid

scintillator.

  • IBD positron is direct proxy for

antineutrino energy

9

0.1% Gd

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SLIDE 10

David Martinez - IIT

IBD Selection

10

① Reject'spontaneous'PMT'light'emission' (“flashers")' ② Prompt'positron:'' 0.7'MeV'<'Ep'<'12'MeV' ③ Delayed'neutron:' 6.0'MeV'<'Ed'<'12'MeV' ④ Neutron'capture'Mme:' 1'μs'<'t'<'200'μs' ⑤ Muon'veto:'

  • Water'pool'muon'(>12'hit'PMTs):'

Reject'[T2μs;'600μs]'

  • AD'muon'(>3000'photoelectrons):'

Reject'[T2'μs;'1400μs]'

  • AD'shower'muon'(>3×105'p.e.):'

Reject'[T2'μs;'0.4s]' ⑥ MulMplicity:'

  • No'addiMonal'promptTlike'signal'

400μs'before'delayed'neutron'

  • No'addiMonal'delayedTlike'signal'

200μs'aaer'delayed'neutron

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SLIDE 11

David Martinez - IIT

IBD Selection

11

① Reject'spontaneous'PMT'light'emission' (“flashers")' ② Prompt'positron:'' 0.7'MeV'<'Ep'<'12'MeV' ③ Delayed'neutron:' 6.0'MeV'<'Ed'<'12'MeV' ④ Neutron'capture'Mme:' 1'μs'<'t'<'200'μs' ⑤ Muon'veto:'

  • Water'pool'muon'(>12'hit'PMTs):'

Reject'[T2μs;'600μs]'

  • AD'muon'(>3000'photoelectrons):'

Reject'[T2'μs;'1400μs]'

  • AD'shower'muon'(>3×105'p.e.):'

Reject'[T2'μs;'0.4s]' ⑥ MulMplicity:'

  • No'addiMonal'promptTlike'signal'

400μs'before'delayed'neutron'

  • No'addiMonal'delayedTlike'signal'

200μs'aaer'delayed'neutron

After this selection on 1230 days


  • f data, we get 2.5 million candidates;


2.2 million from 4 Near Site detectors.

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SLIDE 12

David Martinez - IIT

Daya Bay New results: Fuel evolution analysis

  • DO NOT time integrate: instead,


look at reactors’ fission fractions

  • % of fissions from 235U 239Pu, 238U, 241Pu
  • Calculate ‘effective fission fraction’

  • bserved by each detector:
  • 12

Weight for each of the 6 reactor cores

Basically weight’s each reactor’s fission 
 fraction by distance, power, and oscillation

Daya Bay, Chin. Phys. C 41(1) (2017)

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SLIDE 13

David Martinez - IIT

  • We have fission fractions

and IBDs versus time

  • Let’s compare IBDs


from periods of
 differing effective
 fission fractions!

  • Doing this by combining


periods of common
 fission fraction.

  • We choose 8 bins


in

239 Pu effective 


fission fraction, F239

13

Daya Bay New results: Fuel evolution analysis

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SLIDE 14

David Martinez - IIT

From IBD/day to IBD/fission

  • IBD/day depends on many time-dependent quantities:
  • Reactor status and thermal power
  • Power released per fission
  • Detector livetime
  • Some other more minor, nearly-constant stuff


i.e target mass.

  • Show final plots in terms of IBD/fission
  • Basically take IBD/day and correct for time-dependent

quantities on a week-by-week basis

14

σf

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SLIDE 15

David Martinez - IIT

Results: Flux Evolution

  • When plotting IBD/fission versus

F239, we see a slope in data

  • Very clear that flux is changing with

changing fission fraction.

  • Not too surprising; models predict

239Pu makes fewer νe

  • Seen before in previous

experiments: Rovno (90’s); SONGS (00’s)

15

ROVNO SONGS

  • J. Appl. Phys. 105 064902

Atomic Energy Vol 76 No 2 (1994)

Daya Bay

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SLIDE 16

David Martinez - IIT

16

  • Also consider: total flux prediction is too high by 5.4%
  • Suggest that 235U prediction is too high
  • More complicated scenarios still allowed: 239Pu UP + sterile neutrino.
  • Whatever the case reactor flux models must be wrong in some way.
  • To truly rule out sterile neutrinos, direct tests of L/E with SBL reactor

experiments are required.

Blue line is actually 
 WAY up here!

Results: Flux Evolution

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SLIDE 17

David Martinez - IIT

Result: Fitting For Individual Isotopes

  • Use this data to explicitly fit IBD/fission for

235U, 239Pu

  • Assume loose (10%) uncertainties on sub-

dominant

238U, 241Pu

  • Dominant uncertainties:
  • Statistics
  • Absolute detection efficiency
  • The explanation of

235U only being wrong fits

the data well.

  • 239Pu also matches model well.
  • Note: CLs are significant,


but not overwhelming

  • Future Highly Enriched Uranium (HEU) and

Daya Bay measurements will be necessary for improvements.

17

✔ ✗

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SLIDE 18

David Martinez - IIT

Results: Spectrum Evolution

  • What if we add IBD energy into the mix?
  • Examine evolution in 4 separate

energy ranges

  • Slope is different


for different energy
 ranges.

  • Put another way: IBD


spectrum is changing
 with F239

18

  • This is the first

unambiguous measurement

  • f this behavior
  • Highly relevant to based

nuclear non-proliferation

νe

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SLIDE 19

David Martinez - IIT

Spectrum Evolution: Data-Model Comparison

  • 4-6 MeV region: no strange behavior visible with respect to the

models

  • No major indication that ‘bump’ data-model discrepancy comes

from a particular isotope. Statistics are too low for a meaningful test.

19 Daya Bay, Chin. Phys. C 41(1) (2017)

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SLIDE 20

David Martinez - IIT

  • Would be great to probe a wider range of fission fractions
  • Highly Enriched Uranium cores provide a chance to sample wider fission fraction

ranges.

  • If

235U is to blame, antineutrino flux deficit should be even larger here

  • Enter PROSPECT at highly-enriched

235U HFIR reactor 


in Oak Ridge, Tennessee

20

???

Future: New HEU Measurements

  • T. Langford

Mostly 235U Mostly 239Pu

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SLIDE 21

David Martinez - IIT

Summary

  • Various reasons to question reactor νe models:
  • “The Reactor Antineutrino Anomaly”
  • “Spectrum anomalies”
  • New Daya Bay flux and spectrum evolution results uncover another flaw:

flux evolution is incorrectly predicted.

  • Indicates that incorrect flux predictions are partially responsible for

reactor flux anomaly

  • Upcoming measurements can further clarify this picture:
  • SBL reactor measurements at HEU cores are essential for probing the

nature of the spectral anomaly, and for making conclusive, model- independent tests for sterile neutrinos.

21

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SLIDE 22

David Martinez - IIT

Thanks! Gracias!

22

image A. Obando (http://arturobando.blogspot.com)

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SLIDE 23

David Martinez - IIT

BACKUP

23

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SLIDE 24

David Martinez - IIT

PROSPECT Experimental Layout

  • HEU Reactor: HFIR
  • Segmented liquid scintillator


target region: ~4 tons for
 near detector (Phase I)

  • Moveable: 7-12 m baselines
  • Measure

235U spectrum while directly 


probing sterile oscillations independent of reactor models

24

Sub-cell conceptual design HFIR core shape and
 relative size comparison Near detector conceptual design PMT Light Guide Separator LiLS PROSPECT deployment at HFIR

Phase II:
 far detector moveable Phase I near detector

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SLIDE 25

David Martinez - IIT

Backgrounds

  • b

25

  • Backgrounds make up <2% of 


Near Site IBD candidates

  • Primary background: accidentally


coincident triggers

  • 1.3% of near-site signal;
  • Other backgrounds are


constant over time.

Daya Bay, PRD 95 (2017) Daya Bay, PRD 95 (2017)

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SLIDE 26

David Martinez - IIT

BACKGROUNDS

  • Accidental coincidence between prompt and delayed signals ~1%
  • During detector operation it was found that neutrons from the 241 Am-13 C

calibration sources within the ACUs occasionally introduced several γ rays, correlated in time, to the detector. Contamination from this background was estimated to be ≲0.1%

  • Fast neutrons: Muon interactions in the environment near the detector

generated energetic, or fast neutrons <0.1%

  • 9Li/8He b-n followers produced by cosmic muon spallation. 0.3-0.4%

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SLIDE 27

David Martinez - IIT

The Daya Bay antineutrino detector

3 calibration units per detector. 3 sources per unit: Ge68 (1.02 MeV) Co60 (2.5 MeV) Am241-C13(8 MeV)

27

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SLIDE 28

David Martinez Caicedo AAP2016, University of Liverpool

Antineutrino flux measurement: IBD Yield per nuclear fission(sigma_f) and per GWth (Y)

  • Calculate sigma_f^d for

each detector

  • Md: number of measured IBD events in the d-th

detector with backgrounds subtracted.

  • N_r^f is the predicted number of fissions from the r-th

reactor core

  • E^iso (mean energy release per fission for each

isotope)

  • f^iso_r (average fission fraction of rth core for each

isotope)

  • W_r: average thermal power of r-th core
  • e_d^D : Total detection efficiency
  • Ldr: distance between d-th detector and r-th reactor

core

  • P_sur^dr: Survival probability given and AD-core pair
  • N_d^T : Total number of target protons in the GdLS of

each AD.

Predicted (Huber-Mueller or ILL-Vogel) DATA

28

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SLIDE 29

David Martinez - IIT

IBD candidate rates

29

  • ~ 400-800 IBDs in each near site antineutrino detector per day (x4 ADs)
  • Can see when reactors are turned on and off

Info: 1230-day dataset 
 goes to July 2015

Daya Bay, Chin. Phys. C 41(1) (2017)

Installing 2 more ADs

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SLIDE 30

David Martinez - IIT

Systematics: Detector

  • How does a detector change over time?
  • Reconstructed energy scales are extremely

time-stable (<0.1% variation)

30

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SLIDE 31

David Martinez HEP Seminar- U. Cincinnati

Daya Bay prompt energy spectrum measurement and the “excess” in the 4-6 MeV region

  • Measurements of neutrino capture

time, vertex distributions,delayed energy spectrum of events around 5 MeV region are consitent with the rest

  • f IBD events
  • Theoretical predictions do not

account for the excess of the 4-6 MeV excess in the prompt energy spectrum with a local significance of 4.4 sigma

  • Huber/Mueller prediction can not

described the entire prompt energy spectrum at 2.9 sigma

31 The hatched and red filled bands represent the square-root of diagonal elements of the covariance matrix (sqrt(Vii)) for the reactor related and the full systematic uncertainties, respectively

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SLIDE 32

David Martinez - IIT

Global fit data and results

32

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SLIDE 33

PROSPECT Physics: Absolute Spectrum

HEU Fuel HEU, 4.5% Energy Resolution

  • How much fine structure exists in reactor spectrum?
  • Ab initio calculations suggest significant fine structure from endpoints of

prominent beta branches

  • PROSPECT can


provide highest-ever
 energy resolution


  • n the spectrum
  • Thus, will give best fine 


structure measurement

  • Goal resolution: 4-5%
  • Provide constraints

  • n individual beta branches


(reactor spectroscopy)?

  • Input for next reactor


experiments (JUNO)?

33